Laser-based coating removal systems use pulses of light from high power lasers to ablate or vaporize the paint or other coating from a surface. Ablation is the thermochemical disassociation, vaporization, and removal of material because of the incident convective and radiant energy. FIG. 1 illustrates the surface action phenomena during a typical ablation process.
Commercially-available lasers have the capability to generate pulses of radiant energy for suitable duration to achieve ablation conditions without significant convective heat transfer. Specifically, laser-induced ablation can be limited to the material surface through the appropriate selection of laser wavelength and active control of the radiative heat transfer rate and duration. The important application is the removal of the coatings and finishes from substrate structures that must be protected and reused. The radiative energy input must be significant, but the heat conduction into the substrate must be minimized. The amount of heat conduction into the substrate is affected by the radiant energy and the material surface of the substrate. The wavelength, irradiance, and pulse width of the radiant energy impact the amount of heat conduction into the substrate. The substrate properties, including transmissivity, absorptivity, emissivity, density, specific heat, thermal conductivity, and ablation onset thermochemistry (i.e. melt, vaporization, and surface chemical reaction temperatures), also affect the heat conduction into the substrate.
Wavelength is an important parameter because it must be selected to minimize the transmissivity of the radiant energy into the substrate. Thus, the radiant energy is absorbed at the surface. A high absorptivity of the material surface is also valuable, as this reduces the amount of laser energy needed to achieve a desired ablation rate. In many applications, such as metal substrates below a surface coating, low substrate absorptivity is also advantageous, because this limits the potential for ablation after the surface material is removed. Alternatively, substrate ablation for highly absorbing substrates may need to be limited by the use of active surface recognition prior to the initiation of the laser radiant energy pulse.